This invention relates to a refractory composition characterized by high abrasion resistance and relatively low thermal conductivity, and in particular, to such a composition which may be used as refractory castable and gunning mixes.
Refractory castables are hydraulic setting compositions. They comprise granular refractory aggregates and chemical binders. The refractory castables are shipped in dry form, and when mixed with water to the desired consistency, may be poured like concrete, tamped or rammed into place, troweled or applied with an air gun. Refractory castables take a strong hydraulic set at room temperatures and maintain good strength until the desired ceramic bond is developed as the temperature is increased. Castables are specially suited for furnace linings of irregular contours, for patching brick work and for casting special shapes which may be urgently required. Numerous castable compositions are known, with each of the known compositions having different properties, making each one useful for different applications.
One such application involves the use of refractory castables in lining transfer lines employed in fluid catalytic cracking and fluid coking units used in petrochemical processes. In such units, highly abrasive catalysts or coke travel at high speeds thereby creating extreme erosion potential throughout the catalytic cracking unit. In such units, early abrasion resistant linings were of a dual layer construction consisting of dense hydraulic phosphate bonded refractory facing with insulating back-up which required extensive anchoring and hand ramming to install. To reduce the expense of dual layer linings, the refining industry began using castables of an intermediate density with field additions of stainless steel fibers which required less anchoring in the metal shell, and which could be poured relatively quickly. Although the foregoing improved on the time and cost of installation, lower conductivity was desired.
Abrasion resistant refractory linings in petrochemical vessels are typically chemically hydraulic or phosphate bonded refractory compositions. Abrasion resistance is generally obtained by utilizing a strong, dense refractory grain such as calcined fireclay or tabular aluminum, bauxite, MgAl and a strong bond consisting of aluminum orthophosphate or calcium aluminate cement. In the case of cement, the abrasion resistant bond is achieved by using large amounts of cement, or a combination of fumed silica, cement in amounts less than ten percent, and a surface active agent which allows flow at low water contents. Improved density and subsequent high thermal conductivity values, which are achieved by casting at low water contents results in a highly abrasion resistant bond at low cement levels.
In petrochemical applications, it is desirable that the refractory linings have low thermal conductivity properties. Unfortunately, both density and thermal conductivity generally are directly proportional so that when high bulk density and abrasion resistance are achieved, thermal conductivity is undesirably high. Both density and thermal conductivity may be lowered by the substitution of light weight grain for some of the calcined fireclay, but this has an adverse affect on abrasion resistance.
In addition to cast applications, gunning mixes are used in situations of limited access or where they offer advantages in application efficiency and in associated costs. Dense, homogeneous monolithic linings can be gunned without the use of forms and with a marked savings in time. Gunning mixes used in lining components of fluid catalytic cracking and fluid coking units should possess the same properties of low thermal conductivity and abrasion resistance as the refractory castables.